1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting scheduling information by a User Equipment (UE) in a wireless communication system, and more particularly, to a method and apparatus for transmitting available transmit power information (or a power headroom report) by a UE in a wireless communication system.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd Generation (3G) wireless communication system that is based on Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS), which are asynchronous communication systems, and uses Wideband Code Division Multiple Access (WCDMA). Previously, the 3rd Generation Partnership Project (3GPP) in charge of standardization of the UMTS system proposed a High Speed Downlink Packet Access (HSDPA)/High Speed Uplink Packet Access (HSUPA) system capable of supporting a data rate of up to 10 Mbps in a downlink, as a wireless communication system provided by improving the UMTS system. Further, the HSDPA/HSUPA system has now been commercialized and is now providing services.
Since the proposal of the HSDPA/HSUPA, the 3GPP has more recently suggested a Long Term Evolution (LTE) system as an evolved next-generation wireless communication system. The LTE system will implement high-speed packet-based communication at a data rate of up to 100 Mbps. Accordingly, many studies are underway for commercialization the LTE system.
In both the HSDPA/HSUPA system and LTE system proposed by the 3GPP, a Node B or Evolved Node B (ENB) receives scheduling information from UEs for scheduling the allocation of transmission resources to the UEs. The scheduling information may include, for example, UE buffer status information, available transmit power information, etc.
The buffer status information is information with which the UE informs the ENB of the amount of transmission data in a UE transmission buffer. The available transmit power information is information with which the UE informs the ENB of the amount of power available for uplink transmission. Upon receiving the available transmit power information, the ENB performs scheduling in order to avoid a scheduling error by a UE maximum output limit.
FIG. 1 illustrates a configuration of an LTE system.
Referring to FIG. 1, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified to a 2-node configuration including ENBs 120, 122, 124, 126, and 128, and upper nodes (also known as access gateways) 130 and 132. A UE 101 accesses an Internet Protocol (IP) network 114 through the E-RANs 110 and 112. The ENBs 120 to 128 correspond to legacy Node Bs of an existing UMTS system. Each of the ENBs 120 to 128 is connected to the UE 101 by a wireless channel, and performs complex functions when compared with the function performed by legacy Node Bs.
In the LTE system, because every user traffic including a real-time service, like Voice over IP (VoIP), based on the Internet protocol, is serviced over a shared channel, an apparatus for performing scheduling by collecting status information of UEs is needed, and this scheduling is managed by the ENBs 120 to 128. Generally, one of the ENBs 120 to 128 controls multiple cells. In FIG. 1, it is assumed that ENB 122 is the controlling ENB.
In order to implement a data rate of up to 100 Mbps, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) as a wireless access technology in a bandwidth of up to 20 MHz. In addition, the LTE system uses Adaptive Modulation & Coding (AMC) to determine a modulation scheme and a channel coding rate according to UE channel status.
For example, ENB 122 receives reports on a variety of scheduling information from UE 101 in order to allocate transmission resources to UE 101, i.e., a scheduling operation. Examples of such scheduling information include the buffer status information or Buffer Status Report (BSR) about an amount and type of data UE 101 stores in its transmission buffer, and available transmit power information about available transmit power for UE 101.
The BSR is information indicating an amount of data UE 101 stores in its transmission buffer for uplink transmission, according to the priority. If particular conditions are satisfied, UE 101 generates the BSR and transmits it to ENB 122. For example, the particular conditions may include a situation in which UE 101, which previously had no data stored in it, now has new data to transmit, and a situation in which a predetermined time has elapsed since a previous transmission of the BSR.
The available transmit power information is information indicating an available power that UE 101 can use for uplink data transmission, based on its current channel status. The available transmit power information is transmitted from UE 101 to the ENB 122 using a message of a Medium Access Control (MAC) layer, and is commonly called an Uplink Power Headroom (UPH) or Power Headroom Report (PHR). In the following description, the available transmit power information will be referred to as a PHR.
More specifically, the PHR is defined as a difference between a required transmit power required for uplink transmission and a maximum transmit power for UE 101, when UE 101 performs uplink transmission using the transmission resource and Modulation & Coding Scheme (MCS) level allocated from ENB 122. A method of calculating the PHR may be modified appropriately depending on system conditions.
If the following particular conditions are satisfied, UE 101 generates the PHR and then transmits it to the ENB 122.
Condition 1) A change in a path loss measured by UE 101 exceeds a threshold.
Condition 2) A predetermined time has elapsed since UE 101 transmitted the PHR.
Before performing uplink transmission, UE 101 checks if any one of the above two conditions is satisfied, and if so, includes a PHR in an uplink transmission packet during transmission. In the LTE system, a packet carrying the PHR includes a MAC Protocol Data Unit (PDU).
A PHR generated by Condition 1) is commonly referred to as a regular PHR, and a PHR generated by Condition 2) is commonly referred to as a periodic PHR.
In the LTE system, generation of the periodic PHR is controlled by a timer, i.e., a PHR Periodic Timer. UE 101 starts the PHR Periodic Timer after transmitting a periodic PHR, and when the PHR Periodic Timer expires, UE 101 includes a next periodic PHR in a first MAC PDU being transmitted, and restarts the PHR Periodic Timer. UE 101 repeats this operation.
Additionally, UE 101 resets the MAC in order to receive a handover command from an ENB of the source cell. In the MAC reset process, UE 101 stops all timers in operation, including the PHR Periodic Timer. Because the PHR Periodic Timer is then restarted only when the PHR is generated, the stopped PHR Periodic Timer is not restarted unless a new PHR is generated in a target cell.
Therefore, if only the periodic PHR is set in the UE, excluding the regular PHR, the new PHR is generated only at the expiration of the PHR Periodic Timer. However, because the PHR Periodic Timer was stopped and will not restart, due to the MAC reset as described above, the UE may no longer generate the periodic PHR after its handover to the target cell.